The present invention relates generally to optical networks, and more particularly, to an optical access network with centralized digital optical line termination OLT.
For future optical access networks, there are several architectures under discussion including TDM-PON, WDM-PON, OFDMA-PON and their combinations. Each of them has both pros and cons.
TDM-PON is a simple architecture which uses passive optical splitter to distribute downstream signal to all the optical network unit ONU. At the ONU receiver, all the downstream signals are received and only the data to the desired ONU would be demodulated and forward to the user. For the upstream signal, all ONUs share the same wavelength in different time slots. The schedule of the upstream time slots would be pre-determined by the OLT and broadcasted to all ONUs at the beginning of each scheduling cycle. Time division multiplexing—passive optical network TDM-PON is a low-cost and passive architecture. The deployment is also easy. On the other hand, TDM-PON request burst-mode receiver at the OLT side because of the laser on/off time. Another issue of TDM-PON is the lack of capability for higher data rate (>10 Gb/s) and longer reach (20˜100 km) because of the dispersion, cost/complexity and burst-mode receiver development.
Wave division multiplexing—passive optical networks WDM-PONs use different wavelengths (WDM) for multiple ONUs. Because each ONU has its own wavelength, its downstream and upstream signals can be transmitter independently from other ONUs without generating or receiving any distortion. The challenges of WDM-PON is to use multiple wavelengths in a single system which request multiple transceivers at the OLT. Usually this will increase the cost and system complexity. The difficulty of the deployment will also been increased. Another issue of WDM-PON is the lack of flexibility and it can not share data bandwidth among all ONUs.
Orthogonal frequency division multiplexing—passive optical network OFDMA-PON is a new proposed architecture for next generation optical access networks. Different ONUs can either share the same wavelength or use multiple wavelengths (WDM) for the signal transmission. The major difference is to use OFDMA technologies for multiple access communication. OFDM technology modulate signal on different sub-carriers. When multiple ONUs transmit simultaneously at different sub-carriers. Because all the sub-carriers are orthogonal to each other. There is not any distortion among all ONUs when they transmit at the same time. Due to the OFDM technology, the OFDMA-PON has better Chromatic dispersion tolerance and can be used for long reach PON without any dispersion penalty. However, OFDM technologies request high bandwidth DAC/ADC and high speed DSP modules in the transceiver at both OLT and ONU. This will significantly increase the cost of the whole system.
Accordingly, there is a need for a next generation optical access network with centralized digital OLT.